1. Field of the Invention
The present invention relates to a light source stitching type color image scanning apparatus such as a color image scanner or the like which can read out a color original document.
2. Description of the Prior Art
As conventional apparatus of this type, there is known a line sequential color image scanner which is illustrated in FIG. 1 of the accompanying drawings, for example.
As shown in FIG. 1, a red light source 22, a green light source 23 and a blue light source 24 are disposed under an original document holder 21 made of a transparent glass on which an original document 20 is held. Under the red, green and blue light sources 22, 23 and 24, there are disposed an optical system composed of a mirror 25 and a lens 26 and a CCD (charge-coupled device) linear image sensor 27. This CCD linear image sensor comprises a sensor section formed of an array of a plurality of sensing elements, a read-out gate, a transfer register of CCD structure and an output section connected to the final stage of the transfer register, though not shown.
When a color image is read out by this conventional color image scanner, the red light source 22, the green light source 23 and the blue light source 24 are sequentially energized and reflected lights corresponding to the above respective colors reflected on the original document 20 are introduced into the CCD linear image sensor 27. Then, the CCD linear image sensor 27 operates to sequentially read out the original document 20 at every line on the basis of image signals of respective colors.
A read-out operation is carried out as shown in FIG. 2 of the accompanying drawings.
As shown in FIG. 2, the red light source 22, for example, is energized and a red signal charge is stored in the sensor section. Then, a read-out pulse P1 is applied to the read-out gate to transfer the signal charge to the transfer register. The signal charge is transferred throughout the transfer register by a transfer clock .phi..sub.H to thereby output a red signal from the output section. Then, a remaining signal charge read-out pulse P.sub.2 that is used to read out a remaining red signal charge is applied to the read-out gate to transfer the remaining signal charge to the transfer register. Then, the signal charge of the remaining light is discharged by the transfer clock pulse .phi..sub.H. The green and blue signals will be read out and discharged similarly.
However, according the above example of the prior art, the color image could not be read out at high speed due to the reasons which follow.
That is, in the CCD linear image sensor 27 used in the conventional CCD solid state image sensing devices, the respective light sources 22, 23 and 24 are energized to introduce reflected lights of the original document 20 into the CCD linear image sensor 27, whereby a signal charge is stored in the sensor section. The signal charge thus stored is read out to the transfer register. In this case, as shown by arrows A, B and C in FIG. 2, even after the signal charges are read out, lights are still remaining in the respective light sources 22, 23 and 24 and stored as signal charges. These signal charges must be read out again and then discharged. Therefore, according to the example of the conventional apparatus, when one color signal is read out, a read-out time which results from adding a real signal transfer time and a transfer time of remaining signal charges is needed so that the color image cannot be read out at high speed.